This invention relates to multifocal optical devices which have their multifocal property distributed approximately throughout said devices, and more particularly to multifocal zone plates as described in my Patent Application Ser. No. 970,751 now U.S. Pat. No. 4,210,391.
A multifocal Fresnel lens has been previously described as a lens consisting of a plurality of annular rings, the surfaces of which are made part of a carrier lens, and have the same inclinations to the optical axis, in a repetitive interleaving pattern, as the individual surfaces of the separate single focal power lenses that they wish to combine.
The comcept of a multifocal zone plate was developed as an improvement on the multifocal Fresnel lens, wherein the geometrical spacings of the annular rings of said multifocal Fresnel lens, were set to obey the usual spacing of a zone plate diffraction grating.
Multifocal zone plane mirrors as well as lenses have been described.
However, one can make an improvement in these designs as relates to the quality of the images formed at each of the focal points, by applying phase plate considerations as described by A. I. Tudorovskii in his article "An objective with a phase plate", Optics & Spectroscopy, Feb. 1959. Multifocal phase plates can be designed in such a manner as to cause the light coming to focus at one focal point, to be bright and clear, without the superposition of light passing through zones meant to focus such light to the other focal points.
In order to understand the nature of the problem, we note that in an ordinary monofocal lens all of the incident light comes to focus in phase at the single focal point. However, in an annular zone bifocal Fresnel lens with two focal points, the incident light is shared between the two focal points. For example, we may have half of the incident light pass through the odd zones before being brought to sharp focus at one focal point. But, we will also have, superimposed at this focal point, a blurred image formed by incident light passing through the even zones.